Page:Popular Science Monthly Volume 23.djvu/511

This page has been validated.
495
THE FORMATION OF SEA-WAVES.

storms. The transmission may be effected at very great distances—several hundred miles, for example. It presents a kind of analogy with earthquake-shocks which pass over dead points, and make themselves felt only in places where there are faults or differences in the density of the terrestrial strata.

At Havre, on stormy days, the perturbations of the open sea, transmitted to the shoals of the roadstead, cause oscillations of the water within it, and produce what the sailors call the levée, which, in bad weather, prevents the transatlantic steamers from entering the port. Crossing the entrance of the port, with its breakwaters, the levée penetrates into the outer harbor and spreads out there, attaining two or three metres in amplitude. It enters the Bassin du Roi through a sluice sixteen metres in width, and thence is propagated through a sluice thirteen metres wide to the Bassin du Commerce, where, involved in the ins and outs of the quays, it does not reach more than thirty or fifty centimetres in amplitude. This remarkable phenomenon of the levée, passing into a chain of basins, appears analogous to that of the vibrations of a tense cord divided into sections by a series of frets in contact with it. When we draw a bow over one of the sections of the cord, the others will also vibrate, while a dead point or node will be formed at each place of contact. In the phenomenon under consideration, the entrance of the port and each sluice give rise to a node.

When a mass of water in motion meets an obstacle, it accumulates against it by virtue of its inertia; the water rises, then falls back. This is called the surf, and may be observed along all coasts. It is produced at sea after every tide. The most curious effect induced by it is the back-flow in rivers. The Seine, for instance, flows rapidly at low water; but, as the tide rises, a liquid obstacle several metres high is piled up in less than two hours against the mouth of the river. The water of the Seine then stops, rises, and falls back as surf, while the surf in its turn acts as an obstacle to the current of the river above it. The phenomenon is repeated, and again, and so on, steadily going higher up the river, so that in effect a strong wave ascends the stream. The phenomenon may be easily reproduced on a small scale; every time we suddenly stop the rapid current of a brook or any stream of water, we may see a back-water ascend it.

The amplitude of the movement of waves remains to be spoken of. It appears to be proportional for direct waves to the force of the wind. On the other hand, since each ripple, wavelet, or wave, occupies a given space, and since, as I have already said, a certain number of these are necessary to give rise to a billow or a heavy sea, it is evident that, with a given wind, a billow of a particular dimension can be formed only if a sheet of water extends over a certain area of surface. This is precisely what takes place, and the dimensions of the billows produced by a given wind appear to be proportional to the extent of the